Preheat oven to 350 degrees. Thoroughly grease an ovenproof rectangular pan, about 12-by-8-by-2-inch. Grate the sweet potatoes on the coarse holes of a hand grater or in a food processor (use the shredding disk and then briefly pulse with the steel knife to cut the shreds short), until you have 4 cups. (Save any extra for another use.)

In a large bowl, using a wooden spoon (or in a food processor), cream the sugar with the softened butter. Thoroughly blend in eggs, flour, cinnamon, nutmeg and salt. Stir in sweet potatoes and mix well. If mixture is too dry, add milk.

Turn mixture into the greased pan and bake until golden brown (about 1 hour). Serve hot or at room temperature with sour cream or whipped cream.